This invention relates to processes for the production of poly(1,4-cyclohexylenedimethylene 1,4-cyclohexanedicarboxylate and, more particularly, to processes that have a reduced amount of isomerization of dimethyl trans-1,4-cyclohexanedicarboxylate to dimethyl cis-1,4-cyclohexanedicarboxylate and increased polymerization rates through the addition of certain phosphorus-containing compounds to the polymerization process.
Polyesters of cycloaliphatic diacids and cycloaliphatic diols were first disclosed in U.S. Pat. No. 2,891,930 to Caldwell et al. and are useful in a number of applications, such as in blends with polycarbonate, polyacrylate and other polyesters. U.S. Pat. No. 5,486,562 to Borman et al. discloses blends of poly(alkylene cyclohexanedicarboxylate) and amorphous copolymer resins. U.S. Pat. No. 5,498,668 to Scott discloses blends of an aliphatic or cycloaliphatic polyester with an acrylic polymer. European Patent Application 0 902 052 A1 to Hoefflin et al. discloses an aliphatic polyester-acrylic blend molding composition. Compositions comprising a polycarbonate, a cycloaliphatic resin, an ultraviolet light absorber and a catalyst quencher are disclosed in U.S. Pat. No. 5,907,026, to Factor et al.
Cycloaliphatic polyesters are generally prepared by reacting a cycloaliphatic diol, such as 1,4-cyclohexanedimethanol (CHDM), and a cycloaliphatic diacid or its ester derivative, such as 1,4-dimethylcyclohexanedicarboxylate (DMCD), in a two-stage process typical of linear polyesters. One such process is that described in U.S. Pat. No. 2,465,319 to Whinfield et al. A useful polyester of this type is poly(1,4-cyclohexylenedimethylene 1,4-cyclohexanedicarboxylate), hereafter referred to as PCCD.
In the first stage of the process for preparing PCCD, CHDM and DMCD are reacted in the presence of a suitable catalyst to effect an ester interchange reaction. Ester interchange is typically carried out at temperatures ranging from 180 to 220xc2x0 C. Catalysts that can be used for ester interchange include titanium, lithium, magnesium, calcium, manganese, cobalt, zinc, sodium, rubidium, cesium, strontium, chromium, barium, nickel, cadmium, iron and tin. Normal concentrations of catalyst are in the range of 1 to 500 ppm. Most commonly, titanium is used as the ester exchange catalyst for PCCD. Typically, low molar ratios of diol to diester are used because of the difficulty in the second stage of removing large excesses of high-boiling CHDM diol during polycondensation. Thus, the degree of polymerization that can be obtained in a reasonable length of time is limited (E. V. Martin and C. J. Kibler, pp. 83-134, in xe2x80x9cMan-Made Fibers: Science and Technologyxe2x80x9d, vol. III, edited by Mark, Atlas and Cernia, 1968). A stoichiometric amount of diol to diester can be used, or if appreciable amounts of the diester are lost due to volatilization, a slight molar excess of the diester can be used. The reaction product at the end of ester interchange in the first stage consists of low molecular weight polymer with an average degree of polymerization of about 2 to 10.
In the second stage, polycondensation is effected by advancing the temperature to around 260 to 290xc2x0 C. and applying a vacuum of 0.5 to 1.0 torr to aid in the removal of reaction byproducts. Metals such as titanium, antimony, tin, gallium, niobium, zirconium, aluminum, germanium or lead can be used to catalyze polycondensation and are typically present in the range of 1 to 500 ppm. Most commonly, titanium is used as the polycondensation catalyst for PCCD. Polycondensation can also be carried out in the solid phase. In this procedure, the low-molecular prepolymer is isolated, solidified and granulated. The solid prepolymer is then heated at a temperature about 20 to 40xc2x0 C. below its melting point under a vacuum or in the presence of a flow of nitrogen.
CHDM and DMCD exist as both cis and trans geometric isomers. The equilibrium concentration of isomers in DMCD is 65% trans and 35% cis. DMCD having a trans isomer content greater than the equilibrium concentration can be produced by a number of processes, such as the one described in U.S. Pat. No. 5,231,218 to Sumner et al. For the most useful polymer properties, the starting DMCD used to make PCCD should have a trans content greater than the equilibrium amount of 65%. Preferably, the amount of trans isomer in the starting DMCD monomer is greater than 98% by weight and the amount of cis-isomer is less than 2% by weight. The starting CHDM monomer as supplied typically contains 70% by weight of the trans-isomer and 30% by weight of the cis-isomer. A high level of trans units is desired because incorporation of cis-CHDM or cis-DMCD units into the polymer chain disrupts the chain regularity, lowers the melting point and reduces the amount of crystallinity than can develop in the polymer, as described by Wilfong in J. Polymer Sci., vol. 54, 385-410 (1961).
One disadvantage of the usual process for preparing PCCD is that some of the trans-DMCD units isomerize to the cis-isomer during the polymerization process, thus lowering the melting point of the polymer and reducing the amount of crystallinity in the polymer. The amount of isomerization of the trans-DMCD units that occurs during the polymerization is dependent on several factors, including catalyst type and concentration, reaction temperature and residence time in the reactor. Processes that require a shorter time in the reactor are desirable because there is less time available for the trans-DMCD to undergo isomerization to the cis-isomer. Normally there is no isomerization of the trans-CHDM units during the polymerization process. FIG. 1 illustrates the effect of cis-DMCD units in the polymer chain on the melting point of PCCD. The melting point decreases by about 2xc2x0 C. for every 1 % increase in cis-DMCD units.
U.S. Pat. No. 5,939,519 to Brunelle describes the need for higher crystallinity PCCD. The process requires incorporation of amide segments at up to about 18 mole percent based on total ester and amide segments into PCCD in order to increase the crystallinity, which adds considerable cost to the polymer.
U.S. Pat. No. 6,084,055 to Brunelle discloses a method for the preparation of poly(1,4-cyclohexane dicarboxylates) with maximum molecular weight and crystallinity. The reaction is conducted in a series of progressively increasing temperatures below 265xc2x0 C. with a residence time in the range of 40 to 120 minutes at temperatures above 250xc2x0 C., and/or the reaction is conducted with an initial stage of the reaction in the presence of at least one C2-6 aliphatic diol. While satisfactory results may be obtained using this method, the narrow temperature range and residence time requirements are undesirable because polymerization rates are limited.
U.S. Pat. No. 5,986,040 to Patel et al. discloses crystalline PCCD resins in which the trans-cis ratio of repeating units from DMCD in the polymer is greater than about 6 to 1, and the trans-cis ratio of repeating units derived from CHDM is greater than about 1 to 1 in the polymer. The polyester has a viscosity greater than about 4200 poise and a melting temperature in the range of about 216 to about 230xc2x0 C. A process to produce this polymer is also disclosed. Patel teaches the importance of the starting mole ratio of DMCD to CHDM to control the extent of trans-to-cis isomerization of DMCD. The addition of phosphite compounds to PCCD as color stabilizers is disclosed, although none of the examples indicate that stablilizers were added.
U.S. Pat. No. 5,453,479 to Borman et al. discloses the use of a polyesterification catalyst consisting of a phosphorus compound and a titanium compound to prepare polyesters for blending with polycarbonates. The process advantages are an increased in the strength and mold cycle time of the blend.
The post-reaction addition of phosphite quenchers in blends of polycarbonate, cycloaliphatic polyesters, and ultraviolet light absorbers is disclosed in U.S. Pat. No. 5,907,026 to Factor et al. The phosphite catalyst quencher is added in the post-reaction blending of PCCD with other polymers.
Thus, there exists a need in the art for a fast, simple, cost-effective process for preparing PCCD with a reduced level of isomerization of the trans-DMCD units to cis-DMCD units. Accordingly, it is to the provision of such process that the present invention is primarily directed.
In a process for producing a reactor grade polyester, a poly(1,4-cyclohexylenedimethylene 1,4-cyclohexanedicarboxylate) has a reduced amount of isomerization of the trans-isomer to the cis-isomer of 1,4-dimethylcyclohexanedicarboxylate and an increased polymerization rate by the addition of a phosphorus-containing compound to the reaction process. The process comprises the steps of:
a) reacting a diacid comprising at least 80 mole percent 1,4-cyclohexanedicarboxylic acid or an ester derivative of the diacid comprising at least 80 mole percent 1,4-dimethylcyclohexanedicarboxylate and a glycol comprising at least 80 mole percent 1,4-cyclohexanedimethanol at a temperature sufficient to effect esterification for the diacid or transesterification for the ester derivative;
b) polycondensing the product of step (a) at temperatures and pressures in the presence of a suitable catalyst to effect polycondensation;
c) adding 1 to 800 ppm phosphorus, wherein all parts by weight are based on the weight of the polyester and the phosphorus is added in the form of a phosphorus-containing compound; and
d) after step (c) removing a reactor grade polyester of poly(1,4-cyclohexylenedimethylene 1,4-cyclohexanedicarboxylate) having an inherent viscosity of 0.4 to 2.0 dL/g.
The phosphorus-containing compound is selected from the group consisting of: 
(1) a phosphate ester having the formula:
wherein R1 is a hydrogen atom or a C1-C20 radical, which optionally includes O, Cl or Br atoms, and R2 and R3 are the same C1-C20 radical or a combination of different C1-C20 radicals, which optionally include O, Cl or Br atoms;
(2) a phosphate ester having the formula: 
wherein R is derived from a diol; R1 and R4 can be hydrogen atoms or C1-C20 radicals, which optionally include O, Cl or Br atoms; and R2 and R3 are the same C1-C20 radical or a combination of different C1-C20 radicals, which optionally include O, Cl or Br atoms;
(3) a diphosphate ester having the formula: 
wherein R1 and R4 can be hydrogen atoms or C1-C20 radicals, which optionally include O, Cl or Br atoms, and R2 and R3 are the same C1-C20 radical or a combination of different C1-C20 radicals, which optionally include O, Cl or Br atoms; and
(4) a phosphonate ester having the formula: 
wherein R1 is a hydrogen atom or a C1-C20 radical, which optionally includes O, Cl or Br atoms, and R2 and R3 are the same C1-C20 radical or a combination of different C1-C20 radicals, which optionally include O, Cl or Br atoms.
Further, a reaction product polyester composition of poly(1,4-cyclohexylenedimethylene 1,4-cyclohexanedicarboxylate) is produced having an inherent viscosity of 0.4 to 2.0 dL/g. The polyester composition comprises a diacid component of residues of at least about 80 mole percent of 1,4-cyclohexanedicarboxylic acid, based on 100 mole percent diacid component; a glycol component of residues of at least about 80 mole percent of 1,4-cyclohexanedimethanol, based on 100 mole percent glycol component; 0 to 500 ppm esterification catalyst or 1 to 500 ppm transesterification catalyst; 1 to 500 ppm polycondensation catalyst, and 1 to 800 ppm phosphorus from a phosphorus-containing compound as described above. All parts per weight are based on the weight of the polyester.